In opthalmological devices, fixation targets and/or fixation markers are used for the proper orientation and stabilization of the eye to be examined and/or measured. This is necessary for the execution of measurements with good signal properties and under stable conditions and/or to minimize distorted diagnosis results in case of directional errors.
An important example is the axis length measurement of the eye for facilitating the fitting of an intraocular lens (IOL) for cataract patients, whereby, e.g., the IOLMaster® from Carl Zeiss Meditec AG is applicable. Thereby, it is important that the measurement is executed precisely along the optical axis.
A further example is the repeated OCT measurements of retinal structures, such as nerve fiber layers or edemas for the monitoring of the progress of diseases, such as glaucoma or age-related macular degeneration. Thereto, a sufficiently good fixation is once again required in order to perform comparative measurements.
Even though normal measurement times only amount to a fraction of a second to several seconds, usually 2 sec, many patients have difficulties firmly fixating their eye for the time required for alignment and measurement, which can take up to several minutes. Other patients are not at all capable of firmly fixating their eye. The reasons for that are, e.g.:
Poor eyesight (due to cataract or the like);
Fatigue;
Accommodative difficulties (due to presbyopia or pupil-dilating medication);
Neurological problems (nystagmus or the like);
Uncompensated refractive errors; or
Lack of concentration
However, the incorrect fixation on the displayed fixation markers leads to error-prone measurements. As a result, no sufficient diagnosis can be made or, in the case of cataract patients, no optimal IOL can be selected.
Due to the continued prevailing uncertainty in prior art whether or not the patient is still sufficiently fixated for a measurement after a previous alignment, particularly automated test sequences are made more difficult or repeat measurements become necessary.
According to the known prior art, various solutions are known with which erroneous fixations are to be avoided.
For example, DE 10 2005 003 443 A1 describes a solution for guiding the eye movement of the patent during examination and/or documentation of the eye fundus. Hereby, the fixation marker produced by a spatial light modulator is utilized to align the patient's eye in such a way that the point on the eye fundus to be examined and/or documented is located on the optical axis.
Whether the patient has effectively aligned his/her eye with the fixation marker can thereby only be verified by the operating personnel that simultaneously also observes the eye fundus.
A completely different solution for avoiding erroneous fixation of the eye to be examined on displayed fixation markers is described in DE 10 2009 007 732.4, yet to be published. Hereby, the patient is presented with a preferably variable, fatigue-inhibiting, and attention-fostering fixation marker without making too great a demand on the patient's ability to concentrate. Thereto, the described arrangement exhibits at least one DOE for the modification of the light beam with great luminosity, whereby the light beam is modified in such a way that the resulting beam structure is changeable energetically and/or temporally and/or spatially and/or spectrally.
The suggested method for the display of a fixation marker for opthalmological treatment devices in DE 103 59 239 A1 is also based on a fixation marker, which is supposed to foster the attention of the patient. Hereby, the patient is to align the eye to be treated through foveal focusing with said fixation marker. The fixation marker is moved within the field of vision of the patient, whereby the movement is effected in such a way that the patient can follow the fixation marker without problems. Thereby, the movement of the fixation marker within the field of vision of the patient is effected continuously or incrementally, following either a predetermined or random sequence. As a result, no unwanted eye movements occur. Depending on the type of movement of the fixation marker, measurement or therapy can be effected in different ways. For example, if the fixation marker is moved incrementally within the field of vision of the patient, a diagnosis or therapy preferably takes place only during the short rest periods of the fixation marker. By contrast, with a continuously moving fixation marker, a diagnosis or therapy can also take place while the eye follows the movement of the fixation marker.
A wide range of opthalmological applications and/or individual devices are integrated in the opthalmological imaging device described in US 2007/0291277 A1. For example, in addition to a fundus imaging system, an iris viewer, and an optical coherence tomography system (OCT), the device also exhibits an optical coherence scanning system. For safe handling and attaining exact measurements and/or images, the opthalmological device exhibits various test and fixation markers. The focusing system has the task of aligning the eye through foveal focusing with the displayed fixation marker and immobilizing said eye as safely as possible during the examination and/or imaging. Thereto, fixation markers are produced by the focusing system within the visible wavelength range, for example between 450 and 600 nm, and projected onto the eye. The patient has to align his/her eye firmly with said fixation marker. In the described solution, the fixation system exhibits a display with approximately 120×120 pixels for producing the fixation markers. In addition to an LCD display, other screens can be used for producing the fixation markers. Hereby, the fixation target is variable in size and offers the eye quickly changing visual stimuli.
Whether an exact alignment of the eye to be examined exists in the solutions in accordance with prior art can only be determined by the operator through visual observation of the appropriate alignment of the target area to be examined in the eye, or by the patient confirming the exact alignment with the fixation marker. However, the determination of incorrect measurements can still not be ruled out in both solutions.
As a result, additional, so-called plausibility checks have become prevalent for the known solutions. The recorded measurements are checked for plausibility and consistency in order to eliminate errors in case of incorrect fixation. However, with this method only gross errors are detected since they are simply not plausible. Slight deviations during the alignment with the fixation marker provide measurements which can be incorrect but still plausible.